Grain moisture sensor

ABSTRACT

A meter is disclosed for measuring the dielectric constant of a particulate material, particularly grain, as a function of its moisture content. The meter includes a test cell for containing a sample of the particulate material to be tested. An oscillator includes a variable frequency tuned circuit for varying the frequency of the oscillator output The tuned circuit includes a test cell capacitor using a sample of material to be tested as a dielectric, a calibrated variable standard capacitor and a variable trimming capacitor. An indicator is coupled to the calibrated variable standard capacitor to display a value representing the capacitance setting of the calibrated variable standard capacitor. A frequency monitor compares the output frequency of the oscillator with a selected frequency and a null display displays the difference between the output frequency of the oscillator and the selected frequency. In the invention, the frequency monitor generates DC outputs of opposite polarity according to whether the output frequency of the oscillator is greater than or less than the selected frequency and the null display is a digital display, displaying the frequency difference as zero when the output frequency of the oscillator equals the selected frequency, as a positive value when the output frequency of the oscillator is greater than the predetermined frequency and as a negative value when the output frequency of the oscillator is less than the predetermined frequency.

FIELD OF THE INVENTION

[0001] This invention relates to an improved device for determining certain properties of particulate materials, most particularly the moisture content of grains and similar materials.

BACKGROUND

[0002] The moisture content of grains and like material has a significant impact on the market value of the grain. If grains have too high of moisture content they will sell for less than those in an appropriate moisture range. Therefore, an accurate and precise means of testing moisture content in grain will help farmers monitor their crops. With the aid of monitoring, farmers may dry their grain until the preferred moisture content is achieved. This will minimize energy input while increasing grain value.

[0003] There are many grain moisture sensing devices found in the patent literature. Greenwood et al. Canadian patent 510356, issued Feb. 22, 1955, describes a device allowing a relatively unskilled operator to measure moisture content of a sample rapidly by measuring its dielectric properties. This device has proven to be quite successful in the grain industry and has become a preferred instrument of certain organizations in that industry, for example the Canadian Grain Commission.

[0004] Unfortunately, the manufacture and maintenance of this device has become difficult due to the age of the technology. The device has two vacuum tube oscillators with an operating frequency of several MHz, requiring the use of specially wound inductance coils. Manufacturers have discontinued or are discontinuing production of those components. Hence, it is becoming increasingly more difficult to acquire them for either manufacture or repair. It is clear that the device is in need of an update to use components that are currently readily available.

[0005] In addition to the concerns over manufacture and repair, there are certain deficiencies in the original device. For example, the length of time for temperature stabilization of the electronics and uncertainties in setting a calibrated standard capacitor using an analogue milliammmeter. On the latter point, with the prior art meter, the needle of the ammeter has only positive readings. The capacitor is adjusted to achieve the desired zero current state. The dial reading is positive regardless of whether the capacitance is too high or too low. In adjustment, the dial needle moves to one end of the dial as the measured current output approaches zero. On passing through the point of zero current, the reading begins to rise again, as the needle moves back to the opposite end of the dial. This makes it very difficult to determine the exact point at which the reading is at a minimum and the capacitor is adjusted correctly to provide an accurate reading.

SUMMARY

[0006] According to the present invention there is provided a meter for measuring the dielectric constant of a particulate material to be tested, the meter comprising:

[0007] a test cell for containing a sample of the particulate material to be tested;

[0008] an oscillator including a tuned circuit for varying the frequency of the oscillator output, the tuned circuit comprising:

[0009] a test cell capacitor including two electrodes in the test cell and the sample of particulate material to be tested as a dielectric between the electrodes, such that the capacitance of the test cell capacitor is a function of the dielectric constant of the sample of particulate material;

[0010] a calibrated variable standard capacitor; and

[0011] a variable trimming capacitor;

[0012] an indicator coupled to the calibrated variable standard capacitor to display a value representing the capacitance setting of the calibrated variable standard capacitor;

[0013] a frequency monitor for comparing the output frequency of the oscillator with a selected frequency; and

[0014] a null display for displaying a frequency difference value representing a difference between the output frequency of the oscillator and the selected frequency;

[0015] CHARACTERIZED IN THAT:

[0016] the frequency monitor comprises means for generating DC outputs of opposite polarity according to whether the output frequency of the oscillator is greater than or less than the selected frequency; and

[0017] the null display is a digital display, displaying the frequency difference value as zero when the output frequency of the oscillator equals the selected frequency, as a positive value when the output frequency of the oscillator is greater than the selected frequency and as a negative value when the output frequency of the oscillator is less than the selected frequency.

[0018] The dielectric constant of a sample can be determined as a function of the capacitance of the calibrated variable standard capacitor, as shown on the indicator. The indicator reading is be converted directly to a sample moisture content by reference to standard tabular data.

[0019] Because the frequency difference passes from positive to negative, rather than dipping to a minimum and rising again, it is much easier to detect the desired zero point. This ability is further enhanced with the digital display which provides positive and negative non-zero readings and an unequivocal zero reading at the desired frequency.

[0020] The frequency monitor is preferably a tuned circuit, as opposed to the second oscillator of the prior art. This allows the use of readily available, off-the-shelf components and considerably reduced complexity. Manufacture may be simpler, using printed circuit board technology.

[0021] The device uses the same operating procedure as the prior art device and the same tabular data for the indicator settings so that a standardization of the device with new tabular data for approval by the relevant authorities is unnecessary.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] In the accompanying drawings, which illustrate an exemplary embodiment of the present invention:

[0023]FIG. 1 is a perspective view of a device according to the present invention;

[0024]FIG. 2 is a block diagram of the circuit of the device; and

[0025]FIG. 3 is a more detailed schematic of a preferred embodiment of the circuit.

DETAILED DESCRIPTION

[0026] Referring to the accompanying drawings, there is illustrated a moisture tester 10 having a housing 12 and a test cell 14 for containing a sample of material to be tested. The configuration of the test cell and its mounting and connection to the housing are as described in CA 510 356. The front face 16 of the housing carries a panel 18 on which is mounted a multi-position switch 20 with “off”, “calibrate” and “operate” positions. The panel 18 also carries an indicator dial 22 for a standardized variable capacitor C_(s) (FIG. 3) and a two digit digital display 24. On one side of the housing is a knob 26 for adjusting the standardized variable capacitor with which the dial 22 is associated, while a trim knob 28 is mounted on the opposite side of the housing for adjusting a trimming capacitor. These components have equivalents in the device described in CA 510356, to which the reader is referred.

[0027] Turning to FIG. 2, the device according to the present invention has a regulated power supply 30. This is the source of all power used in the device. An oscillator 32 is coupled to a tuned circuit 34 that is used for adjusting the frequency of the oscillator output. The oscillator output is delivered to a frequency monitor 36, which produces a DC output 38 as input to an analogue to digital converter 40. The output from converter 40 serves as the input to the digital display 24.

[0028] Referring to the more detailed circuit drawing in FIG. 3, it will be seen that the oscillator 32 in this embodiment is an Armstrong tuned-gate oscillator using a field effect transistor (FET) 42. The tuned circuit 34 includes inductor L₂ and capacitors C₂, C_(v), C_(c), C_(s), C_(t), and C_(cal). This provides an oscillating voltage to the gate of the FET through a blocking capacitor C₁ when excited. The drain current through inductor L₁, the tickler coil, varies sympathetically. It is inductively coupled to inductor L₂ of the tuned circuit to provide the requisite feedback. Variations in the capacitance of circuit 34 change the frequency of oscillation. For present purposes a frequency of about 18 MHz is appropriate. This circuit is “series-fed”, with the bias and the signal flowing in the same drain circuit.

[0029] The power supply 30 includes the power source 44, conveniently a battery, the three pole, double throw switch 20 and a voltage regulator 46. The regulator is shunted with capacitors C₉ and C₁₀ to provide a good ground for the signal.

[0030] In the off position of switch 20, the power supply is disconnected. When it is moved to the “calibrate” position, power is delivered to the drain circuit and the circuit to relay switch 48 is completed to close the switch and connect capacitor C_(Cal) in parallel with capacitors C₂, C_(v), C_(c), C_(s), C_(t), and inductor L₂ for initial calibration of the meter. In the “operate” position of the switch, relay switch 48 is open and the calibration capacitor C_(Cal) is disconnected from the tuned circuit 34.

[0031] The drain circuit signal is passed through capacitor C₃ to the frequency monitor 36. The frequency monitor includes a second tuned circuit, itself including two inductor coils L₃ and L₄, both connected to the oscillator output and connected in series with an intervening capacitor C₄. Inductor L₃ is connected to the cathode of a diode D₁, with its anode grounded. Inductor L₄ is connected to the cathode of a diode D₂, with its anode connected to a voltage divider consisting of resistors R3 and R₄. The AC output of the circuit 36 is grounded through capacitor C₅. Appropriate selection of the coils L₃ and L₄ along with the associated components in circuit 36 yields a circuit tat acts as an electronic “teeter totter” or “see saw”. At one particular frequency, (about 18 MHz), the coils produce voltages that cancel one another so that the DC circuit output is zero. As the input frequency increases, the voltage of coil L₃ increases, while that of L₄ decreases, yielding a positive output voltage. With decreasing frequency, the voltage of coil L₄ increases and that of L₃ decreases, yielding a negative output voltage.

[0032] The DC output from circuit 36 goes to A/D converter 40 and thence to digital display 24. The resistors R5 and R6 balance the converter 40 to yield a “0” output with a 0 volt input.

[0033] The operating procedure for the device is the same as the prior art device, with the exception of the relatively long warm-up time that is not required with the new apparatus. Initially, with the test cell 14 empty, the switch 20 is set to the “calibrate” position to connect the capacitor C_(cal). The standardized variable capacitor C_(s) is then adjusted to produce a predetermined reading on the indicator 22. The trimming capacitor C_(t) is then adjusted to produce a null or zero output on the display 24. At this point, the frequency of the oscillator matches that of the tuned frequency monitor circuit. The switch 20 is then set to the “operate” position, taking the calibrating capacitor C_(cal) out of the circuit. A sample of material to be tested is then added to the test cell. This alters the test cell capacitor C_(c) and with it the oscillator frequency, resulting in a non-zero output from the frequency monitor.

[0034] To bring the oscillator frequency back to that of the tuned frequency monitor circuit, the standardized variable capacitor C_(s) is adjusted to produce a zero reading on the display 24. The reading on the indicator dial 24 may then be compared to standard tabular data to determine the moisture content of the material, as done with the prior art.

[0035] While one embodiment of the present invention has been described in the foregoing, it is to be understood that other embodiments are possible within the scope of the invention and the invention is to be considered limited solely by the scope of the appended claims. Potential modifications within the abilities of those skilled in the art include: the use of different forms of variable frequency oscillator, for example a Hartley, Colpilts or Clapp oscillator; the use of an alternative to the FET as the amplifying component of the oscillator; and the use of a different frequency monitor circuit. The design described in the foregoing is currently preferred as it integrates well into the existing prior art meter for refurbishing or repair. It also uses the same operating process and tabulated standard data so that the transition from the prior art unit to the new one is, for the operator, straight forward. 

1. A meter for measuring the dielectric constant of a particulate material to be tested, the meter comprising: a test cell for containing a sample of the particulate material to be tested; an oscillator including a tuned circuit for varying the frequency of the oscillator output, the tuned circuit comprising: a test cell capacitor including two electrodes in the test cell and the sample of particulate material to be tested as a dielectric between the electrodes, such that the capacitance of the test cell capacitor is a function of the dielectric constant of the sample of particulate material; a calibrated variable standard capacitor; and a variable trimming capacitor; an indicator coupled to the calibrated variable standard capacitor to display a value representing the capacitance setting of the calibrated variable standard capacitor; a frequency monitor for comparing the output frequency of the oscillator with a selected frequency; and a null display for displaying a frequency difference value representing a difference between the output frequency of the oscillator and the selected frequency; CHARACTERIZED IN THAT: the frequency monitor comprises means for generating DC outputs of opposite polarity according to whether the output frequency of the oscillator is greater than or less than the selected frequency; and the null display is a digital display, displaying the frequency difference value as zero when the output frequency of the oscillator equals the selected frequency, as a positive value when the output frequency of the oscillator is greater than the selected frequency and as a negative value when the output frequency of the oscillator is less than the selected frequency.
 2. A meter according to claim 1 wherein the frequency monitor comprises a tuned circuit including two reactance elements connected to produce respective DC voltages of opposite polarity.
 3. A meter according to claim 2 wherein the two reactance elements are inductors. 